As a method for producing a bonded SOI (Silicon On Insulator) wafer using the bonding method, there is conventionally known a technique comprising bonding two of silicon wafers via a silicon oxide film, for example, a method comprising forming an oxide film on at least one of such wafers, bonding the wafers to each other without interposing foreign matters between the surfaces to be bonded, and then subjecting them to a heat treatment at a temperature of 200-1200° C. to enhance the bonding strength, as disclosed in Japanese Patent Publication (Kokoku) No. 5-46086.
The bonded wafer, of which bonding strength is enhanced by such a heat treatment, can be subjected to subsequent grinding and polishing processes. Therefore, an SOI layer for fabricating devices can be formed by reducing thickness of the wafer on which devices are to be fabricated to a desired thickness by grinding and polishing.
A bonded SOI wafer produced as described above has advantages of superior crystallinity of the SOI layer and high reliability of a buried oxide layer existing directly under the SOI layer. However, because it is produced through reduction of thickness of the wafer by grinding and polishing, it suffers from a drawback that the reduction of thickness takes a lot of time and generates waste of the material. In addition, the uniformity of the obtainable film thickness is only in such a degree of target thickness ±0.3 μm or so at most.
Meanwhile, in connection with recent use of higher integration degree and higher processing velocity of semiconductor devices, further reduction of film thickness and improvement of film thickness uniformity are required as for the thickness of the SOI layer. Specifically, a film thickness and uniformity represented as 0.1±0.01 μm or so are required.
A thin film SOI wafer having such a film thickness and film thickness uniformity cannot be realized in a bonded wafer by the conventional thickness reduction processing using only grinding and polishing. Therefore, the method called ion implantation and delamination method or hydrogen ion delamination method was developed as a novel film thickness reduction technique as disclosed in Japanese Patent Laid-open (Kokai) Publication No. 5-211128.
This ion implantation and delamination method is a technique for producing an SOI wafer, wherein an oxide film is formed on at least one of two silicon wafers, hydrogen ions or rare gas ions are implanted into one wafer (also called “bond wafer”) from its top surface to form a micro bubble layer (enclosed layer) in the silicon wafer, then the ion-implanted surface of the wafer is bonded to the other wafer (also called “base wafer”) via the oxide film, thereafter one of the wafers is delaminated as a thin film at the micro bubble layer as a cleavage plane (delaminating plane) by subjecting them to a heat treatment (delamination heat treatment), and the bonded wafer is further subjected to a heat treatment (bonding heat treatment) to strengthen the bonding to obtain an SOI wafer.
In this method, the delaminated plane is obtained as a good mirror surface, and an SOI wafer having extremely high uniformity of the film thickness of SOI layer can be obtained relatively easily. In addition, the method also has an advantage that one delaminated wafer can be reused and thus the material can be used effectively.
Further, by this method, it is also possible to directly bond silicon wafers to each other without an oxide film, and it can be used not only for a case where silicon wafers are bonded to each other, but also for a case where an ion-implanted silicon wafer is bonded to an insulator wafer having a different thermal expansion coefficient such as those of quartz, sapphire, silicon nitride, aluminum nitride and so forth or an ion-implanted insulator wafer is bonded to another wafer to obtain a wafer having a thin film of such materials and so forth.
Further, there has also recently been developed a technique, although it is a kind of the ion implantation and delamination method, in which hydrogen ions to be implanted are excited and implanted in a plasma state so as to enable the delamination process at room temperature.
Although this ion implantation and delamination method is an extremely excellent method as a method for producing a bonded SOI wafer, it is necessary to reduce bonding failures called voids, which are generated at the bonded interface, in order to produce such an SOI wafer at a level of mass production with good yield.
It was already elucidated that the major cause of the generation of the voids, which are likely to be generated in a usual bonded wafer produced by not using the ion implantation and delamination method, resides in particles adhered to the bonding surface. According to Japanese Patent No. 2675691, it is described that, if size of the particle is 0.5 μm or more, voids are generated. That is, if such particles exist on the bonded surface when two wafers are bonded, unbonded portions (voids) will be formed on the bonded interface. Since, as for the size of the voids, the voids have an approximately circular shape having a diameter of about 0.5 millimeter to several tens of millimeters, they can be observed by X-ray topography, ultrasonic reflectoscope, infrared interferometry or the like, even for wafers as they are after the bonding at room temperature or those subjected to a heat treatment thereafter for enhancing the bonding. Therefore, in order to reduce such voids, the bonding can be performed after the wafers are subjected to cleaning for removing the particles adhering to the surfaces to be bonded to remove the particles as much as possible.
However, in case of a bonded wafer produced by using the ion implantation and delamination method, when the bonded wafer surface after the delamination process was precisely observed, it was found that, even if voids having a diameter of about 0.5 millimeter to several tens of millimeters were not generated at all, which are observed in a usual bonded wafer, there may be generated bonding failure portions (called “micro-voids” hereinafter) having a size markedly smaller than the size specified above (a diameter of several micrometers to several tens of micrometers, or even smaller than that).